The subject of the invention concerns the technical area of rotary magnetic sensors, of the type that includes a coder element moving close to a detection cell and designed to read off at least one angular position in the general sense.
The subject of the invention finds a particularly advantageous application in the automobile area in which such a sensor can be used, for example, in the context of ignition functions.
In the above preferred area, it is common to produce a magnetic sensor designed to measure changes in the intensity of a magnetic field, when a coder equipped with a series of elements generating a variable magnetic field moves in front of one or more measurement or detection cells. Each detection cell, such as, for example, a Hall-Effect or magneto-resistive probe, delivers a periodic electrical signal corresponding to changes in the intensity of the magnetic field generated by the elements. Each detection cell is associated with a hysteresis type level comparator, such as Schmitt trigger, in order to obtain sharp transitions of the output voltage for distinct values of the magnetic field, according to whether it varies upwards or downwards.
In order to constitute a sensor for detection of a speed of rotation, it is common to create a coder that is equipped with elements that generate a variable magnetic field, arranged in a regular manner along a circumference.
According to a first form of implementation, the generating elements are composed of elements that disrupt a magnetic field created by a fixed magnet placed close to such disruptive elements. As an example, such disruptive elements are composed of teeth arranged in a ferromagnetic ring.
According to a second form of implementation, the generating elements of a variable magnetic field are formed by magnetic poles, regularly spaced with a given pitch. Such a coder thus comes in the form of a multipolar magnetic ring.
To allow the determination of at least one position, corresponding, for example, to the top dead centre point in the ignition of a cylinder, it is common to create a marker on the magnetic coder which is commonly known as a singular or irregular pole. It is also common, for example, to remove two teeth from the ferromagnetic ring. In the implementation variant using a coder made in the form of a multipolar magnetic ring, it is possible to envisage either removing several magnetic poles leaving an empty space, or replacing one or more poles of a given polarity with one or more poles of an opposite polarity. One thus creates a given magnetising pole which has, between its two adjacent poles of opposite polarity, a spacing pitch that differs from the spacing pitch of the other poles.
Such a sensor thus delivers a digital output signal with a regular coding part with a constant period and an irregular coding part with a different and longer period. In practice, this leads to a difficulty for the exploitation of such a signal given, in particular, its non-periodic character at the level of the singular pole.